This invention relates to timing systems for vehicle engines. The invention has particular application in systems that include two camshafts. The dual camshafts can both be rotated by connection to a single chain driven by the engine. Often, one of the camshafts is driven directly by the engine and the second camshaft is driven by an additional camshaft-to-camshaft ("cam-to-cam") chain drive. This drive generally comprises a pair of sprockets mounted on the driving shaft and driven shaft; a drive chain engaging both sprockets, and an intermediate tensioning device.
A tensioning device, such as a hydraulic tensioner, is used as a control device for the timing system. As a chain transmits power from a driving sprocket to a driven sprocket, one portion or strand of the chain between the sprockets will be tight while the other portion of the chain will be slack. In order to impart and maintain a certain degree of tension in the slack portion of the chain, a hydraulic tensioner provides a piston that presses against a tensioner arm or other chain guiding mechanism.
Prevention of excess slack in the chain is particularly important in the case of a chain driven camshaft in an internal combustion engine in that a chain without sufficient tension can skip a tooth or otherwise throw off the camshaft timing, possibly causing damage or rendering the engine inoperative. However, in the harsh environment of an internal combustion engine, various factors can cause fluctuations in the chain tension.
For instance, wide variations in temperature and thermal expansion coefficients among the various parts of the engine can cause the chain tension to vary between excessively high or low levels. During prolonged use, wear to the components of the power transmission system can cause a decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations cause considerable variations in chain tension. Reverse rotation of an engine, occurring for example in stopping or in failed attempts at starting, can also cause fluctuations in chain tension. For these reasons, a mechanism such as a hydraulic tensioner is desired to ensure the necessary tension on the slack side of the chain.
Hydraulic tensioners are a common method of maintaining proper chain tension. In general, these devices employ a tensioner arm or lever arm that pushes against the chain on the slack side of the chain. This lever arm must push toward the chain, tightening the chain when the chain is slack, and must provide resistive force when the chain tightens.
Typically, a hydraulic tensioner includes a piston in the form of a hollow cylinder. The piston slides within a bore in the housing and is biased outward from the housing in the direction of the tensioner arm and chain by a piston spring. The interior of the piston forms a high pressure fluid chamber with the bore or opening in the housing. The high pressure chamber is connected through a one way check valve to a low pressure chamber or reservoir, which provides or is connected to an exterior source of hydraulic fluid.
Upon start-up, the force of the spring on the piston causes the piston to move further outward as the chain begins to move. Outward movement of the piston creates a low pressure condition in the high pressure fluid chamber, or pressure differential across the inlet check valve. Accordingly, the inlet check valve opens and permits the flow of fluid from the reservoir, or low pressure chamber, into the high pressure chamber. When the high pressure chamber is sufficiently filled with fluid, the force on the chain that moves the piston inward will be balanced by the outward force from the spring and the resistance force of the fluid in the chamber. The force of the chain against the fluid in the chamber also causes the check valve to close, which prevents further addition of fluid to the chamber.
In the typical tensioner and chain system, the various elements are separately assembled and then must be combined in the finished system on the engine. The assembly of these elements each separate from the other and separate positioning and mounting of each element results in the possibility of errors, a relatively long assembling time and possible differences in assembling, and therefore in performance, between one engine and another. The present invention is directed to an integrated assembly that overcomes these disadvantages.